1. Field of the Invention
The invention discloses new drilling fluids based on water-based O/W-emulsions and O/W-emulsion drilling muds based thereon, which are distinguished by high ecological acceptability and at the same time good standing and application properties. An important area of use for the new drilling mud systems is in off-shore wells for the development of petroleum and/or natural gas deposits, the aim of the invention being particularly to make available drilling fluids with high ecological acceptability which can be used in industry. The use of the new drilling mud systems admittedly has particular significance in the marine environment, but is not limited thereto. The new mud systems can also be put to quite general use in land-based drilling, i.e. also for the development of petroleum and/or natural gas deposits here. They are, however, new valuable working agents, for example, also in geothermal wells, in water bore-holes, in the drilling of geoscientific bores and in drilling for the mining industry. It is also essentially true here that the associated ecotoxic problems are substantially simplified by the new water-based O/W-drilling fluids selected according to the invention.
2. Discussion of Related Art
Liquid mud systems used in the sinking of rock bores for bringing up the loosened drill cuttings are known to be flowable systems, thickened to a limited extent, which can be assigned to one of the three following classes:
Purely aqueous drilling fluids; drilling mud systems based on oil, which as a rule are used in the form of so-called invert emulsion muds, and represent preparations of the W/O-emulsion type in which the aqueous phase is distributed as a heterogeneous fine dispersion in the continuous oil phase. The third class of the known drilling fluids is composed of water-based O/W-emulsions, i.e. fluid systems which contain a heterogeneous, finely-dispersed oil phase in a continuous aqueous phase. The invention discloses improved systems of this latter type. The application properties of the drilling fluids of such O/W-emulsion systems take an intermediate position between the purely aqueous systems and the oil-based invert fluids. The advantages, but also the disadvantages, of the purely aqueous systems are connected with the advantages and disadvantages of the oil-based invert-emulsions disclosed hitherto. Detailed information on this subject can be found in the relevant specialist literature, refer, for example, to the text book by George R. Gray and H. C. H. Darley, "Composition and Properties of Oil Well Drilling Fluids", 4th. edition, 1980/1981, Gulf Publishing Company, Houston, and the extensive specialist and patent literature cited therein and to the manual "Applied Drilling Engineering", Adam T. Bourgoyne, Jr. et al., First Printing Society of Petroleum Engineers, Richardson, Tex. (USA).
One of the main weaknesses of purely water-based drilling mud systems lies in the interaction of water-sensitive, particularly water-swellable, rock and/or salt formations with the aqueous drilling fluid and the secondary effects initiated thereby, in particular bore-hole instability and thickening of the drilling fluid. Many proposals are concerned with the reduction of this problem area and have, for example, resulted in the development of the so-called inhibitive water-based muds, cf. for example, "Applied Drilling Engineering", loc. cit., Chapter 2, Drilling Fluids, 2.4 and Gray and Darley loc. cit., Chapter 2, in particular the sub-section on pages 50 to 62 (Muds for "Heaving Shale", Muds for Deep Holes, Non-Dispersed Polymer Muds, Inhibited Muds: Potassium Compounds).
In more recent practice, in particular drilling fluids based on oil, which consist of the 3-phase system oil, water and finely particulate solids and are preparations of the W/O-emulsion type, have succeeded in overcoming the difficulties described above. Oil-based drilling fluids were originally based on diesel oil fractions containing aromatics. For detoxification and to reduce the ecological problems created thereby, it was then proposed to use hydrocarbon fractions which are largely free of aromatics--now also known as "non-polluting oils"--as the continuous oil phase, see in this regard, for example, the publications by E. A. Boyd et al. "New Base Oil Used in Low Toxicity Oil Muds", Journal of Petroleum Technology, 1985, 137-143 and R. B. Bennet "New Drilling Fluid Technology--Mineral Oil Mud", Journal of Petroleum Technology, 1984, 975-981 and the literature cited therein.
Drilling fluids of the water-based O/W-emulsion system type have also hitherto used pure hydrocarbon oils as the dispersed oil phase, cf. here, for example, Gray, Darley loc. cit., p. 51/52 under the section "Oil Emulsion Muds" and the tabular summary on p. 25 (Tables 1-3) with details of water-based emulsion fluids of the salt-water mud, lime mud, gyp mud and CL-CLS mud type.
In this context in particular it is known that water-based O/W-emulsion fluids represent a substantial improvement in many regards to the purely water-based drilling mud systems. Particularly in more recent times, however, the advantages and disadvantages of such water-based emulsion fluids have also been examined critically in comparison with the oil-based invert-systems. This is due to the considerable ecological reservations felt towards the oil-based invert drilling fluids commonly used today.
These ecological reservations can be subdivided into two problem areas:
In addition to the basic constituents, i.e. oil and water, all drilling fluid systems based on water and/or oil require a large number of additives for the establishment of the desired application properties. The following can be mentioned here purely by way of example: emulsifiers or emulsifier systems, weighting agents, fluid-loss additives, wetting agents, alkali reserves, viscosity regulators, in some cases auxiliary agents for the inhibition of drilled rock with high water-sensitivity, disinfectants and the like. A detailed summary can be found, for example, in Gray and Darley, loc. cit., Chapter 11, "Drilling Fluid Components". The industry has developed additives which currently appear ecologically harmless, but also additives which are ecologically questionable or even ecologically undesirable.
The second problem area is caused by the oil phases used in such drilling fluids. Even the hydrocarbon fractions which are largely free from aromatics, currently known as "non-polluting oils", are not completely harmless when released into the environment. A further reduction in the environmental problems, which are caused by the fluid oil phases of the type referred to here, appears urgently necessary. This is true in particular for the sinking of off-shore wells, e.g., for the development of petroleum or natural gas deposits, because the marine eco-system reacts particularly sensitively to the introduction of toxic and poorly degradable substances.
There have recently been some proposals for reducing these latter problems. For example, the U.S. Pat. Nos. 4,374,737 and 4,481,121 disclose oil-based invert-drilling fluids in which non-polluting oils are to be used. The following can be mentioned together as of equal value as non-polluting oils: mineral oil fractions which are free from aromatics and vegetable oils, such as peanut oil, soybean oil, linseed oil, corn oil, rice oil or even oils of animal origin, such as whale oil. These named ester oils of vegetable and animal origin are all, without exception, triglycerides of natural fatty acids, which are known to have high environmental acceptability and are clearly superior from the ecological point of view to hydrocarbon fractions--even when these do not contain aromatic hydrocarbons.
In the above U.S. Patent Specifications, however, not one concrete example describes the use of such natural ester oils in invert drilling fluids. Without exception, mineral oil fractions are used as the continuous oil-phase. In fact, oils of vegetable and/or animal origin of the type mentioned here are not considered for invert drilling fluids for practical reasons. The rheological properties of such oil phases cannot be controlled over the wide temperature range generally required in practice, from 0.degree. to 5.degree. C. on the one hand, up to 250.degree. C. and more on the other.